1. Field of the Invention
The present invention generally relates to information recording methods and information recording apparatuses for phase change optical disks such as DVD-RWs (ReWritable), which include a recording layer whose phase is reversibly changed between a crystal phase and an amorphous phase and are compatible with the formats of, for example, DVD-Videos (Digital Videos or Digital Versatile Discs) and playback-only DVDs such as DVD-ROMs.
2. Description of the Related Art
With the penetration of multi-media, various information recording disks have been developed: examples include playback-only disks such as DVD-Videos and DVD-ROMs, recordable DVD-Rs using an organic dye material for a recording layer, and rewritable DVD-RWs using a phase change material for a recording layer.
Information (sectors, in this example) recorded on such DVDs is in the format shown in FIG. 1A. In such a format, data (sectors) are continuously recorded on the entire track of a disk at a constant linear density as shown in FIG. 1B.
In order to achieve an information recording medium having compatibility with playback-only disks, information is recorded with a constant recording channel clock frequency as shown in FIG. 1D while constantly maintaining the linear velocity in the track by controlling the rotational speed of the disk such that the number of revolutions becomes inversely proportional to the track radius by using a CLV (Constant Linear Velocity) method as a method for controlling the rotational speed of the information recording medium (disk) as shown in FIG. 1C.
However, in order to control the rotational speed by the CLV method, it is necessary to vary the rotational speed of a disk such that the linear velocity in the track becomes constant. That is, a great running torque and a large and high-cost motor are required since the speed of a spindle motor for rotating the disk needs to be varied. In addition, there is a disadvantage in that, at the time of seeking, a longer access time is required compared to that of an HDD or a MO drive, since a waiting time period is required until completion of speed variation of the spindle motor.
In order to perform recording on a disk while maintaining a constant rotational speed of the disk without performing speed variation control, it has also been conceived to make the format of information recorded on a disk as shown in FIGS. 2A through 2D. That is, as shown in FIG. 2D, the frequency of a channel clock used in recording of the disk is decreased in the inner tracks and increased in the outer tracks such that the frequency is proportional to the radius position of the track. In this case, since the recording linear velocity becomes small in the inner tracks and large in the outer tracks, the recording linear density is constant as shown in FIG. 2B. In addition, it is possible to record information on the disk while maintaining a constant number of revolutions (rotational speed) of the disk as shown in FIG. 2C, i.e., by using a CAV (Constant Angular Velocity) method.
Accordingly, it becomes unnecessary to perform rotational speed variation control of the spindle motor that rotates the disk. Thus, low-revolution torque will suffice and it is possible to use a small and low-cost motor. Further, since speed variation is not performed, the waiting time period at the time of seeking is eliminated. Consequently, it is possible to significantly reduce the access time period.
Additionally, it is also possible to use a ZCLV method in which an optical disk medium is divided into a plurality of regions (zones) in the radial direction thereof, and the recording speed for each of the zones is varied such that the average number of revolutions (angular velocity) of the disk over the zones becomes constant while maintaining a constant recording linear velocity in each of the zones by using the above-mentioned CLV method.
However, generally, in a phase change optical disk medium, the ratio among the pulse width of a recording pulse sequence, heating power, and erase power of a laser emission during recording at a specific recording linear velocity is optimized, and the states of marks and spaces formed vary at a different recording linear velocity. In other words, the heat capacity of a top heating pulse, which is necessary for formation of a mark, may be excessive or deficient, the average length of marks may be different due to variation in cooling rate, and faulty erasing and degradation of a recording film may occur due to excessive or deficient erase power. Hence, jitter may be degraded or the number of times of overwriting may be decreased.
In this regard, according to Japanese Laid-Open Patent Application No. 5-22570, in order to obtain in a relatively short time period an optimum recording light volume corresponding to the entire recordable regions of each optical disk, the optimum light volume is obtained at the same recording linear velocity for each of at least two positions in a test-writing region. Then, by performing interpolation or extrapolation with respect to the optimum recording light volumes at the two recording linear velocities obtained by an interpolating routine, the optimum recording light volumes are obtained for all of the recording linear velocities.
In addition, according to Japanese Laid-Open Patent Application No. 5-274678, in order to reduce the laser power required for recording without degrading jitter characteristics, in a method of recording information in the outer track regions at frequencies higher than those for the inner track regions by emitting an optical beam whose intensity is modulated in accordance with an information signal on the basis of a reference clock that is different for each region while rotating an optical disk at a constant number of revolutions, the optical beam is periodically emitted at a frequency that is an integral multiple of the frequency of the reference clock for each region. Additionally, when the optical beam is emitted on the outer track regions, the duty ratio of pulse emission is set larger than that in the time when the optical beam is emitted on the inner track regions.
Further, according to Japanese Laid-Open Patent Application No. 10-106008, in order to provide an optical disk apparatus capable of high-speed and highly-reliable recording, an optical disk, an optical head, synchronization signal generating means, a VCO, phase comparing means, a controller, and recording signal generating means are provided, and the height and width of a pulse of a recording signal is varied in accordance with the recording linear velocity, such that recording can be consistently performed under the best recording conditions.
Additionally, Japanese Laid-Open Patent Application No. 2001-118245 discloses a method in which, among a top heating pulse duty ratio Ttop for varying the front edge of the top heating pulse in a recording pulse sequence, an end-off cooling pulse duty ratio Tecp for varying the rear edge of the end-off cooling pulse in the recording pulse sequence, and an erase power ratio E that is the ratio of an erase power Pe with respect to a heating power Pw, at least two of the above-mentioned set values are updated at predetermined intervals. Thereby, even if the recording linear velocity is varied by CAV control that makes the number of revolutions of a disk constant, recording is performed in which uniform characteristics are obtained over the entire surface of a phase change optical disk medium.
However, generally, it is difficult for a phase change optical disk to correspond to a wide-range recording linear velocity. Particularly, in a case where recording is performed at a high recording linear velocity, it is necessary to adjust the length of a recording mark by lowering the level of erase power. Thus, there is a problem in that erase power sufficient for overwriting cannot be supplied.
In this regard, according to Japanese Patent Publication No. 02707774, it is disclosed that overwriting characteristics at a high recording speed are improved by composing an erasing pulse part by multiple pulses, thereby achieving the erasing level of high erase power and reduction of the average power by using low erase power. However, in a recording state where the recording linear velocity is not constant as in CAV recording, it is difficult to perform recording in which uniform signal characteristics are obtained over the entire surface of a disk and a decrease in the number of times of overwriting is prevented.
That is, in the above-mentioned patent documents, the set values of some elements of a recording pulse sequence, such as the duty ratio of pulse emission, are varied in accordance with the recording linear velocity as in the CAV method. However, merely qualitative effects with respect to an optical disk medium are obtained, which are not sufficient for recording disks, particularly, for DVDs.
In other words, since variation in overwriting characteristics, i.e., variation in characteristics of recording information (RF signal) such as a decrease in the number of times of overwriting, is not only affected by formation of a recording mark part but also by the light emission waveform of an erasing pulse part in an interacting manner, the recording methods disclosed in the above-mentioned patent documents are not sufficient, and it is not always possible to obtain uniform signal characteristics over the entire surface of an optical disk and to avoid a decrease in the number of times of overwriting. Thus, it is not always possible to obtain desired effects at a high recording linear velocity covering a wide range. Moreover, even if the set values of an erasing pulse part are to be varied, quantitative study has not been conducted with respect to how to vary the set values.